Having established that the silicate transport in intact cells and membrane vesicles of the diatom, Nitzschia alba, is sodium gradient-dependent, we shall attempt to isolate and characterize the biochemical components of the transport system, and to study the mechanism of its regulation. We shall also continue our studies on the role of silicate in the synthesis of the nuclear DNA polymerases of Cylindrotheca fusiformis by quantifying the silicate-dependent specific changes in the total messenger RNA population. Also, since we have found a relationship between the availability of silicate and levels of cyclic AMP and cyclic GMP in C. fusiformis, we shall investigate the effect of silicate on the activities of adenylate cyclase, guanylate cyclase, and the phosphodiesterases.